A power converter for use with photovoltaic cells is disclosed, or, more particularly, individual DC/DC micro-converters for dedicated use with at least one photovoltaic module are disclosed.
As photovoltaic (PV) solar power installations continue to increase in number and in scale, harvesting and managing power efficiently has become more challenging. Equally as challenging is the management of PV power installations on a national level via a “smart grid”. In particular, it is desirable to increase the demand for renewable energy, to supplement and/or replace energy produced via fossil fuels. Enhancing PV power use, however, requires reduction in the production cost per kilowatt hour and reduction in utility transaction costs for PV interconnections.
For the latter, traditional PV power generating and control systems use at least one of centralized inverters, bipolar centralized inverters, string inverters, and micro-inverters. Conventionally, DC/AC inverters have been used to extract maximum power from PV systems that include arrays formed by plural PV modules connected in series and parallel configurations and to convert the unregulated generated DC power to grid-voltage, synchronized AC power. The AC power generated can be transmitted and distributed either directly to AC loads or through distribution transformers. According to this traditional approach, low-voltage DC power transfer concerns and simplicity of power conversion options necessitate configuring the PV modules in serial strings and/or in parallel string arrays. However, the deleterious effects of shading, soiling, and other lighting degradation on individual PV modules and, hence, PV module characteristics matching require greater consideration.
Referring to FIG. 1, for a photovoltaic array 10 to achieve its highest energy yield and greatest efficiency, current practice includes carefully matching the electrical characteristics of each PV module 15 in each series-connected string 12 and of each parallel-connected string 14. Matching creates considerable labor and expense during manufacture at the factory. More problematically, even if PV modules 15 are ideally matched at the time of manufacture, a single PV module 15 in any string 12 can quickly degrade the performance, i.e., DC output, of the entire PV array 10. Indeed, decreasing the current or voltage output from a single PV module 15 degrades the output of the entire string 12 of series-connected PV modules 15, which has a multiplied effect on the performance of the entire PV array 10. This is especially true when direct sunlight is blocked from all or some portion of one of more of the PV modules 15.
For example, if the amount or intensity of sunlight striking a discrete PV module 15 is blocked, for example, due to shading, e.g., from clouds, vegetation, man-made structures, accumulated moisture, and the like, or due to soiling, i.e., contamination with soil or other organic or non-organic matter, then even ideally matched PV modules 15 perform poorly. Moreover, the affected PV module(s) 15 may suffer from excessive heating.
When centralized inverters 13 are used, output from plural PV modules 15 that are structured and arranged in strings 12 of parallel rows 14 of strings 12 is combined and processed. Power optimization and conditioning is, consequently, performed on the combined DC input.
Advantageously, these systems are highly evolved and reliable and, moreover, they facilitate centralized communication, control, and management through the centralized inverter 13. Disadvantageously, there is no PV string level management or control. Hence, overall array performance is still adversely affected by underperforming individual strings. Indeed, panel mismatch resulting from, inter alia, shading, soiling, and the like, reduces efficiency.
Traditionally, bypass diodes 16 and blocking diodes 18 are adapted to deal with the variability (matching) of discrete, individual PV modules 15 and with solar irradiance. More specifically, to minimize degradation of the total DC output of the array 10 that may result from mismatch or differences in the voltage or current outputs of discrete PV modules 15, bypass diodes 16 can be integrated with each PV module 15. When forward biased, the bypass diodes 16 provide an alternate current path around an underperforming PV module 15. Bypassing the underperforming PV module 15 ensures that the string's 12 voltage and current outputs are not limited by the voltage and current output of the underperforming PV module 15. Disadvantageously, bypassing the underperforming PV module 15 reduces the string's 12 voltage output by, effectively, taking the underperforming PV module 15 off-line.
Similarly, blocking diodes 18 can be integrated with strings of series-connected PV modules 12 in the PV array 10. When the total voltage output from a string of series-connected PV modules 12 exceeds a biasing voltage associated with the blocking diode 18, the DC voltage output is fed onto the DC bus for transmission to the inverter 13. However, if the total voltage output from the string of series-connected PV modules 12 is less than the biasing voltage associated with the blocking diode 18, then the blocking diode 18 is not forward biased and, hence, voltage output from the string 12 is blocked from going the DC bus.
Bi-polar centralized inverters are slightly more efficient than uni-polar centralized inverters. Advantageously, bi-polar applications tend to be cheaper, lighter in weight, and do not suffer from transformer losses, simply because they do not include a transformer. Disadvantageously, as with centralized inverters, there is no PV string level management or control, which, along with parallel processing and panel mismatch, reduces efficiency. Bias voltages may also be introduced in the array. Furthermore, although the inverters themselves do not include transformer circuitry, a transformer is still required to step up the power delivered to a commercial or utility grid.
To avoid reliance on bypass diodes 16 and blocking diodes 18, one approach has been to connect PV modules 15 to DC/AC micro-inverter(s). DC/AC micro-inverters are known to the art and embody the finest-grained configuration in which maximum possible power can be extracted from each PV module 15 regardless of mismatch, soiling, shading, and/or aging. For the purposes of this disclosure, “micro-inverters” will refer to inverters that perform a DC to AC power conversion and “micro-converters” (introduced below) will refer to converters that perform a DC to DC power conversion.
Micro-inverters are adapted to reduce mismatch and other losses by converting DC power to AC power locally, e.g., at each PV module 15 or cell and/or at every PV string 12 in the PV array 10, which facilitates string-level management. Micro-inverters have proven effective for small systems that yield higher total kilowatt hours (kWh). Disadvantageously, micro-inverters involve complex electronics that may require sophisticated cooling. Moreover, large-scale applications may require servicing and maintaining hundreds—if not thousands—of units, which have not yet been engineered to operate dependably for 20 years or more.
Multi-phase AC systems also need to be configured from single phase units, requiring appropriate transformer step-up to utilization and/or to distribution voltages. Moreover, although generating single phase AC power, the micro-inverter has double line frequency energy storage requirements. This generally causes either a significant ripple current through the PV module 15—which reduces yield—or requires utilization of electrolytic capacitors. Electrolytic capacitors, however, are unreliable and the acknowledged “Achilles heel” of any power conversion system that utilizes them.
Furthermore, integrating energy storage into a PV array 10 with micro-inverters is not straightforward. For example, because the DC node is internal to each of the micro-inverters, each energy storage system requires a discrete, dedicated micro-inverter. The issue of grid interaction and control can be daunting with so many devices in parallel.
Current practice needs with micro-inverters also include additional electronics, which normally are located in a hot environment, which is to say, on the reverse side (back) of the PV module 15. The ambient environment on the back of a PV module 15 is not particularly conducive to long life of the electronics, having an operating range as high as 80° C.
The challenges facing DC to DC micro-converter applications include achieving a highly reliable, lower-installed cost per Watt system that provides increased kWh yields. Such systems should provide centralized and de-centralized monitoring and control features; should include electronics that can be controlled locally or remotely, to react to variable array and grid conditions; and that can be easily integrated with a commercial or utility grid.
U.S. Pat. No. 6,127,621 to Simburger discloses a power sphere for a spinning satellite that purportedly minimizes mismatch losses on the solar cells by providing individual DC/DC “regulators” for each individual solar cell, to regulate the power delivered to a load. U.S. Pat. No. 6,966,184 to Toyomura, et al. discloses a PV power-generating apparatus having power conversion devices individually connected to solar cell elements to convert the output of the elements. The plural DC/DC converters are connected in parallel and are operated so that changes in the input voltage to a DC/AC inverter move the operating point of the solar cell element, which changes the input voltage to the DC/DC converters. In this manner, input voltage to the DC/AC inverter from each converter is controlled to be the same.
U.S. Pat. No. 7,193,872 to Siri discloses a power supply having an inverter for connecting plural DC power sources to a utility grid using a single DC/DC conversion stage. The Siri system purports to control current based on feed-forward compensation as some function of an input power commanding voltage (VERR). More specifically, the current and voltage from a solar array are sampled from which the input power commanding voltage is output. A current reference generator generates a reference current (IREF) which is the product of the input power commanding voltage, an instantaneous utility line voltage, and the inverted square of the VRMS signal.
A photovoltaic power system that includes plural photovoltaic strings or an array of power-generating photovoltaic modules and a controller therefor that provide PV string level control, to regulate and stabilize output voltage of each PV string individually, to harvest greater energy and increase kWh produced is desirable.
Means for integrating replacement modules into a PV array without having to match the electrical properties of the replacement module to those of the modules already in the array is also very desirable.